Speaker cabinet

ABSTRACT

A smaller-sized speaker cabinet developed in this invention is made of a mixture that is composed of a woody cellulose powder of proper physical properties and a synthetic resin blended with this powder. The resin is a non-chlorinated resin, and the cellulose powder consists of particles whose diameters are included in a range from about 5 μm to 500 μm. The cellulose powder is surface-treated to increase affinity for the resin, so that efficient manufacture of the cabinets of improved acoustic performance is now possible.

INDUSTRIAL FIELD OF THE INVENTION

The present invention relates to a speaker cabinet that is designed toenclose the rear and side faces of a speaker body in order to raise itsacoustic radiation efficiency.

BACKGROUND OF THE INVENTION

Every speaker has been accommodated in a cabinet, as widely known in theart. In case of large-sized high-grade speaker cabinets, they areusually made of woody materials. In case of other speaker cabinets ofrelatively smaller sizes, variety of their configuration as well aseasiness to mold them are important factors. Therefore, either pieces ofsynthetic resins mixed or not-mixed with any fillers, or molded metalpieces, are used as such smaller speaker cabinets. An example of suchmolded smaller-sized cabinets is shown in FIG. 1 that is an illustrationof the first embodiment of present invention. This cabinet is usuallyformed as a box (1), that has in one of its faces a hole (13) throughwhich a speaker held in this box does face outwards. This face (12)having the hole formed therein is commonly called a frontal plate (orbaffle plate), and in a few cases, it is made integral with another part(11) of this box. In most cases, such smaller-sized speaker cabinetshave each the frontal plate (12) manufactured as a discrete member ineach cabinet. In a proper region that may be a back face (not shown) ofthe speaker cabinet box, an input terminal (not shown) is disposed toreceive a signal current from an amplifier.

Acoustic waves radiated forwards from a speaker not enclosed in anycabinet will vary their phase every moment and continuously, butopposite to that of synchronous acoustic waves radiated backwards fromthis speaker. Due to this phenomenon, those forward and backward waveswill naturally tend to partially cancel one another and lower theradiation efficiency. Therefore, speakers have each been installed inthe hole (13) of a closed type cabinet so that a diaphragm of thatspeaker might keep the interior thereof isolated from the exterior. Inthe so-called “pass-ref” type cabinets, an acoustic circuit comprising adiaphragm, ports and the like will acoustically separate the interiorfrom exterior. Mutual cancellation of acoustic waves opposed indirection is avoided in these ways, lest radiation efficiency should beimpaired.

In another case wherein the speaker is placed in a cabinet, not only theenergy of backward acoustic waves from this speaker, but also avibration energy transmitted from its frame will force the cabinet wallsto vibrate and produce extra acoustic waves. Such extra waves from thosewalls will be sensed as a non-linear and extraordinary distortion inregenerated sounds of an impaired quality. Frequencies and intensitiesof the distorted sound will vary depending on the shape, structure andconstructive material of the cabinet. Physical properties such asdensity, Young's modulus, tan δ (viz., mechanical internal loss) and thelike are particularly important to the material of cabinets. Thesefactors should be optimized to diminish vibration of their walls beingdesigned.

An object of the present invention is to resolve these problems by meansof an improved material of speaker cabinets. Certain powdery cellulosesubstances of excellent physical properties will be employed herein todevelop a novel mixture. Those cellulose substances may be blended withcertain synthetic resins to enable an efficient and inexpensivemanufacture of smaller-sized speaker cabinets of a much better acousticperformance.

DISCLOSURE OF THE INVENTION

In order to achieve the object noted above, the present invention willprovide a speaker cabinet composed of a mixture of a non-chlorinatedresin and a cellulose powder, wherein the powder consists of particleswhose diameters are included in a range from about 5 μm to 500 μm.

This mixture may be molded to form the speaker cabinet, and the diameterrange is more preferably from about 4 μm to about 400 μm. Thenon-chlorinated resin may either be a thermosetting resin, or athermoplastic resin that is typically a polyolefin resin, a polystyreneresin, a polyester resin or the like. Among these resins, apolypropylene resin is most preferable because of its higher mechanicalstrength, its easiness to process and its cleanliness unlikely to causeany environmental pollution when discarded. The cellulose powder may beminute woody particles, minute paper particles, powdery pulp, powderycotton linter or the like. Surface treatment of these celluloseparticles is desirable in order to give their surfaces an improvedaffinity to the resins, possibly forming chemical bonds with the latter.Content by weight of the cellulose powder in the mixture may preferablybe about 30%-70%. An excessive content of cellulose powder will lowerrigidity of the molded article, rendering it so fragile as inadequate insome cases.

The surface treatment of cellulose powder necessitated herein to improveits affinity to resins may for example be esterification, that will bedone using a polybasic acid anhydride such as maleic anhydride. Arelatively small amount of an organic peroxide such as benzoyl peroxidewill contribute to further raise the affinity of esterified cellulosepowder for synthetic resins. In this case, such a cellulose powderhaving an excellent affinity for the resins can be blended therewith atany desired high rate within the range noted above, without impairingfluidity of a resultant mixture. Preparation of the mixture capable ofbeing smoothly molded is thus made easier. Even if a polyolefin isemployed as the resin, surfaces of resultant products may simply beground in a mechanical manner so as to enable them to be painted withany colorant and/or bonded to any foreign articles. It is a matter ofcourse that any usual plasma spray coating or primer coating may be doneto further improve the bond strength of final products and/or stabilityof their coated membranes.

Among the thermoplastic resins and thermosetting resins as thenon-chlorinated resins that are blended with a cellulose powder in theinvention, polyolefin resins are the most preferable materials. Inparticular, polypropylenes are best, owing to their easiness to mold,better physical properties, readiness to recycle, easiness of disposaltreatments, cheapness and the like features. Injection molding is thetypical technique of molding conducted herein, although extrusion maysubstitute for it. Any conventional press may be employed in the moldingprocess that will preferably be carried out within a range oftemperatures from about 160° C. to 200° C. A proper quantity of anypigment or synthetic colorant of a desired hue may be added to themixture so as to provide cabinets that need not be colored later. If awoody powder of cypress is used as the cellulose powder and a mixturethereof is molded at a temperature controlled as noted above, thenresultant products will remain fragrant for a long time. Any syntheticperfumery may alternatively be used in pretreatment of cellulose powderto be controlled as to its aroma.

By virtue of such surface-treated particles of the cellulose powder, ithas a significantly increased affinity for the resin molecules containedin the mixture. This feature enables the cellulose powder to mix freelyin any remarkably high proportion, raising rigidity of the products.Frequently, a bending stress and strain may be observed in the cabinetwalls vibrating. However, a high internal loss (i.e., tan δ) of said thecellulose powder and the intrinsic internal loss of the resin will besummed up to raise the overall internal loss of such highly rigid walls.Speakers installed in such cabinets of the invention will never causeany noticeable resonance in the cabinet walls. Even if any intensivesignal pulses do force these cabinet walls to vibrate at extremely highamplitude, their sharp vibration will be attenuated and damped soon notto impair the quality of regenerated sounds to any considerable degree.

Additionally, excellent fluidity of the mixture is afforded herein asnoted above, by virtue of improved affinity of the cellulose powder andresin molecules for each other. Owing to this feature, any conventionaltype of injection molding machines can be used free from any surplus inequipment investment and any change in the molding conditions. Higherratios in content of cellulose powder will neither adversely affectfluidity nor manufacture efficiency. In contrast with a case of usingsome inorganic harder filling agents, the molding machine will beprotected well from early damage or wear and tear of its molds,cylinders and screws, in favor of inexpensive maintenance.

Further, the mixed materials forming the boxes in products of theinvention can be recycled. Possible incineration of them will scarcelygive rise to the problem of environment pollution, since thenon-chlorinated resins does not produce any noticeable amount of toxicgases or harmful ashes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a speaker cabinet provided in a firstembodiment of the present invention;

FIGS. 2(a), 2(b) and 2(c) are graphs of transient overshoot in vibrationobserved in some speaker cabinets including one that is shown in FIG. 1;

FIG. 3 also is a graph of the relationship between temperatures and dataof Young's modulus that were measured on the cabinet walls shown in FIG.1; and

FIG. 4 is another graph of the relationship between temperatures andvalues of tan δ that were measured on the cabinet walls shown in FIG. 1.

BEST MODES OF CARRYING OUT THE INVENTION

Now, reference is made to FIG. 1 that is a perspective view of a speakercabinet 1 of a first embodiment. This cabinet 1 box-shaped as usualconsists of a cabinet body 11 and a frontal lid 12. Both the body andlid are bolted with self-tapping screws to a core not shown, so as toprovide a closed box. A hole 13 formed in the frontal lid 12 is forfixture of a speaker. Although not shown, input terminals for receivingsignal currents are disposed in the rear face of this cabinet. Leads 14for connection to the speaker have inner ends previously attached to theinput terminals. Reinforcement ribs 15 are formed on and integral withthe inner face of the present cabinet.

The speaker cabinet of the embodiment is made from a mixture commposedof (1) 49.5% by weight of a polypropylene resin as the non-chlorinatedsynthetic resin, (2) 50% by weight of a modified woody powder as thecellulose powder, and (3) 0.5% by weight of benzoyl peroxide as theorganic peroxide. The modified woody powder has been surface-esterifieddue to reaction of 10 parts by weight of maleic anhydride with 100 partsby weight of raw woody powder having an average particle size of 100 μm.This mixture is molded into a speaker shown in FIG. 1, at a targettemperature within a range of from about 160° C. to 200° C. The speakercabinet has a dimension of 170 mm×170 mm×120 mm, respectively for itsheight, width and depth. Thickness and density of cabinet walls are 4 mmand 1.10 gr/cc, respectively.

A speaker having a diameter of 12 cm and installed in an example of thecabinet of the invention was driven with current signals, and transientexcessive vibration of its walls was inspected and analyzed to give aresult shown in FIG. 2(a). FIG. 2(b) shows one corresponding date thatwas obtained in another cabinet, which had been prepared as a referenceor control made of a polypropylene (wall thickness and density being 4mm and 0.9 gr/cc, respectively). FIG. 2(c) likewise shows the othercorresponding date that was obtained in still another cabinet as anotherreference made of a middle density woody fiber board (i.e., MDF board)whose wall thickness and density were 9 mm and 0.69 gr/cc, respectively.The ordinate in each graph of transient excessive vibration of thecabinet walls does indicate the output voltage from a vibration pickupor vibration sensor. Voltage peak appearing in FIG. 2(a) in theinvention was about 0.67 times (−3.4 dB) as strong as that appearing inFIG. 2(b) given herein as a reference case. Such a difference inintensity of the voltage representing the cabinet wall vibration can bedetected well by human ears as such decreased distortion in regeneratedsounds.

In FIG. 3, the solid curve (a) represents Young's modulus E′ as afunction of temperature, for the speaker cabinet of the embodiment. Thedash-and-dot curve (b) represents the corresponding data found in thereference cabinet made of polypropylene. Temperature was varied within arange of 0° C. to 50° C., taking into account the condition under whichthe speakers will operate.

In FIG. 4, the solid and dash-and-dot curves (a) and (b) represent thedata of tan δ as a function of temperature, respectively for the cabinetof the embodiment and for the reference cabinet of polypropylene. Itwill now be apparent that any change in ambient temperature within therange noted above does scarcely affect the modulus of, or internal lossof mechanical energy in, the material of cabinet of the invention. Itsvibration will never show any significant change that would adverselyaffect the quality of regenerated sounds from season to season.

The above embodiments do not delimit the scope of invention but may bemodified for example by changing the materials of speaker cabinet to anyequivalent ones, by changing the molding apparatus as to its heatingmeans or the like, insofar as the structural features, the objects andadvantages of the present invention are ensured or achieved.

UTILIZABILITY IN THE INDUSTRIES

The present speaker cabinet is composed of a mixture comprising acellulose powder and a synthetic resin such that the surface layers ofpowder particles are esterified to increase affinity for the resin. Arelatively high content of this powder in the mixture raises rigidity ofand increases internal loss in the cabinet walls. Speakers installed insuch cabinets will scarcely cause resonance, and even if intensivesignal pulses cause vibration of cabinet walls, it will be attenuatedsoon not to impair quality of regenerated sounds. Good fluidity of themixture results from improved affinity of the cellulose powder for theresin. Any conventional injection machine is usable without changing themolding conditions but suppressing equipment investment. Higher ratiosof cellulose powder will not affect fluidity of the mixture ormanufacture efficiency. Due to non-use of inorganic filling agents, themolding machine is now protected from early damage of its molds,cylinders and screws, lowering maintenance cost. Although the mixedmaterials can be recycled, incineration of them will not causeenvironmental pollution, since the non-chlorinated resins do not produceany noticeable amount of toxic gases or ashes. The present speakercabinet is thus highly advantageous from many points of view.

1. A speaker cabinet composed of a mixture of a non-chlorinated resinand a cellulose powder, wherein the powder consists of particles whosediameters are included in a range from about 5 μm to 500 μm.
 2. Aspeaker cabinet as defined in claim 1, wherein content by weight of thecellulose powder in the mixture is from about 30% to 70%.
 3. A speakercabinet as defined in claim 1, wherein the resin is selected from thegroup consisting of polyolefin resins, polyester resins, polyesterresins and polystyrene resins.
 4. A speaker cabinet as defined in claim1, wherein the cellulose powder is surface-treated to increase affinityfor the non-chlorinated resin.
 5. A speaker cabinet as defined claim 1,wherein the mixture is colored with a colorant.
 6. A speaker cabinet asdefined in claim 1, wherein the cellulose powder has an intrinsicperfume, and the mixture is molded at a temperature within a range from160° C. to 200° C.
 7. A speaker cabinet as defined in claim 1, whereinthe cellulose powder is surface-esterified using a polybasic acidanhydride so as to increase affinity for the non-chlorinated resin.
 8. Aspeaker cabinet as defined in claim 1, wherein the mixture of thenon-chlorinated resin and a esterified cellulose powder furthercomprises an organic peroxide.